When a fogging generated by moisture aggregating on and adhering to a glass surface due to temperature change or increase in humidity on a vehicle windshield occurs in a wide range of a visual field area for securing a visual field of a driver (hereinafter referred to as a “visual field area”), this adversely affects driving. Accordingly, an antifogging means suppressing occurrence of a fogging in a vehicle, such as, specifically, a means for sending wind to a windshield surface, a means for directly adjusting a glass temperature such as a glass having a heat wire, or the like is provided, and a response to a fogging is made by operating this means. Further, adjusting air conditioning, or the like is also a typical response made for antifogging.
These responses for antifogging are normally started in a stage that a fogging starts to occur, and when the timing to start the antifogging response delays, the fogging spreads through the visual field area and hinders driving. Accordingly, if an initial state of a fogging to occur on the vehicle windshield can be detected and the timing to start the antifogging response can be made early, the visual field area can be maintained constantly in a favorable state. From this viewpoint, for example, a method and a device for detecting a fogging on the vehicle windshield described in Patent Reference 1 (JP-A H10-048124) have been developed. However, they require optical measurement and hence it cannot be said that they are simple and able to accurately detect a fog.
On the other hand, there has been proposed use of a glass provided with an antifogging film for vehicle windshield by providing an antifogging film including a water absorbing resin, for example, a water absorbing crosslinked resin layer obtained from polyepoxides, on a glass surface and absorbing and removing minute water droplets which cause a fogging (See Patent Reference 2 (JP-A 2008-0273076)).
With such a glass with an antifogging film, occurrence of fogging on the windshield can be suppressed or delayed to a certain level, but a fogging occurs when moisture aggregates and adheres onto the windshield and exceeds the water absorbing ability of the antifogging film. Accordingly, when the glass with the antifogging film is used for a vehicle windshield, chances of the fogging to occur and the antifogging response to be carried out decrease, but when the fogging occurs once, the typical response for antifogging is carried out similarly to the above. At this time, when the timing to start the antifogging response delays, the above problem that the fogging spreads through the visual field area and the driving is hindered can similarly occur.